1. Field of the Invention
The present invention relates to a steel strip wherein the steel strip comprises two to seven round steel filaments lying in parallel adjacent to one another so as to form a single plane of core filaments and one or more filaments which are wrapped around the core filaments in one single wrapping direction.
2. Description of the Related Art
Such steel strips may be used for the reinforcement of elastomers or rubber products, e.g. rubber tires. Particularly, such steel strips may be used for the reinforcement of plies in a radial tire, whereby the term plies refers to carcass plies or to breaker plies. The elongated cross-section of such steel strips is highly appreciated in the plies of a radial tire: the steel strip has a higher bending stiffness in the plane of the strip than in a plane comprising the longitudinal axis of the strip and being perpendicular to the plane of the strip. The high bending stiffness in the plane of the strip reduces the deformation and heat generation under alternating stresses in that plane whereas a good flexibility is maintained in any plane perpendicular to the plane of the strip. Moreover, such a steel strip provides the amount of steel required without decreasing the fatigue resistance or without increasing the ply thickness, on the contrary.
In case such a steel strip is used for the reinforcement of plies in a rubber tire, the diameter of the core filaments ranges from 0.10 mm to 0.40 mm, and preferably from 0.15 mm to 0.35 mm. The core filaments preferably have a tensile strength above EQU 2325-1130 log d Newton/mm.sup.2
whereby d is the filament diameter expressed in mm. The composition of the steel filaments has a carbon content which ranges from 0.60 to 0.95%, preferably from 0.75 to 0.85%, a manganese content between 0.40 and 0.70%, a silicon content between 0.15 and 0.50% and a maximum sulphur content and phosphorus content of 0.03%, all percentages being percentages by weight. The composition may also comprise--either alone or in combination --other alloying elements such as chromium, molybdenum, vanadium, nickel, boron (either free or bound), niobium . . . The core filaments are conveniently covered with a rubber adherable coating such as brass or a brass alloy. The filament which is wrapped around the core filaments usually has a diameter which is smaller than the diameter of the core filaments. This filament is not necessarily a round filament, it may have a flattened cross-section. This filament is not necessarily a steel filament with the same composition as mentioned hereabove, but may be any filament which unifies the core filaments and gives the steel strip the necessary compression resistance.
When the steel strip is used for the reinforcement of plies in a radial tire, it must not have any flare, it must be straight and must be flat over the whole length of the strip. The steel strip must not have any wildness so that calendering and cutting does not form any problems.
Due to the asymmetrical action of the wrapping filament(s) which is (are) wrapped around the core filaments, however, the requirements of flatness and absence of wildness and flare over the complete length of the strip have not been met by existing steel strips.